In recent years, optical disc devices have increased in capacity by virtue of a shorter wavelength of a light source, a larger NA of a lens and the like. For a shorter wavelength of a light source, digital versatile discs (DVDs) adopt an AlGaInP based red semiconductor laser at 650 nm for realizing information reproduction with a higher density, whereas compact disc devices adopt near-infrared light at 780 nm. In order to realize a next-generation optical disc device with a still higher density, the development of practical blue light sources have become imperative.
As one measure for realizing a blue laser, a quasi-phase-matched second-harmonic-generation (hereinafter abbreviated as QPM-SHG) technology is available, which is an optical waveguide quasi phase matching system using a wavelength conversion device. Compared with direct-emission type GaN based blue semiconductor lasers receiving attention recently, a SHG blue laser employing the QPM-SHG technology has the merits of low noise (−145 dB/Hz) and a small variation in wavelength and divergence angle, and a small driving voltage (2 V) of an AlGaAs based semiconductor laser that is the source of the fundamental wave. In order to allow a SHG blue laser to be used as a light source for optical discs, such a laser must be reduced in size and weight. In this regard, since a planar type direct-coupled SHG blue laser does not require a lens for a coupling system, sufficient miniaturization can be realized (See for example, JP 3156444 B, pp 4 to 6, FIG. 7).
FIG. 13 shows a configuration of a planar type direct-coupled SHG blue laser module. On a Si sub-mount 1, an optical waveguide QPM-SHG device 2 and a wavelength tunable DBR semiconductor laser 3 are mounted. The SHG device 2 is composed of a ridge type optical waveguide 5 and a periodically domain-inverted region 6 that are formed on an X-cut MgO-doped LiNbO3 substrate 4. The LiNbO3 substrate 4 is joined to the sub-mount 1 by an adhesive layer 7 made of an ultraviolet-curing adhesive. The sub-mount 1 is joined to a package 9 shaped like a box with an Ag paste 8. In a wall of the package 9, an aperture 9a is provided for letting light output from the SHG device 2 outside.
The SHG device 2 is configured so that a difference in propagation speed between a fundamental wave light and a second harmonic light generated by the semiconductor laser 3 can be compensated by the periodically domain-inverted region 6 so as to satisfy a quasi phase matching condition. Since the fundamental wave and the harmonics propagate through the ridge type optical waveguide 5 as guided waves, a long interaction length can be ensured, thus realizing a high conversion efficiency.
In order to allow such a SHG blue laser module to be smaller, lighter and have a lower cost, the miniaturization of various components is required. Then, an attempt has been made for the miniaturization of a SHG blue laser module, where the device width of the LiNbO3 substrate 4 constituting the SHG device 2 was reduced from 3 mm to 0.85 mm and the width and the thickness of the Si sub-mount 1 were reduced from 5 mm to 2 mm and from 0.8 mm to 0.3 mm, respectively.
As a result of the miniaturization, the SHG device 2, the sub-mount 1 and the package 9 became vulnerable to an influence of swelling when the temperature increases. Because of the narrow device width of the SHG device 2, the bonding area of the adhesive layer 7 is reduced in the width direction, resulting in deterioration of the adhesive strength. Further, as a result of the decrease in thickness of the sub-mount 1, the sub-mount 1 becomes easy to bend even under a small stress, thus further increasing a tendency to generate misalignment of coupling between the SHG device 2 and the semiconductor laser 3 during a temperature change as compared with that in the conventional device size. The coupling misalignment during a temperature change occurs not only at the time of securing to the sub-mount 1 but also during various reliability tests for a module, such as a heat cycle test, a high temperature holding test and a high temperature and high humidity test.
In the optical waveguide wavelength conversion type SHG device, a blue light power as a harmonic power is proportional to the square of a coupled power of infrared light as fundamental wave, and therefore deterioration of the coupled power of the infrared light due to the coupling misalignment causes considerable deterioration of the output of the blue light. The coupling misalignment due to a temperature change in the operating environment also becomes likely to occur, so that temperature characteristics of the SHG blue laser are affected. That is to say, during the high temperature operation, e.g., during the operation at 60° C., deterioration of the output of the blue light becomes remarkable due to deterioration of the coupling efficiency between the semiconductor laser and the SHG device.
The deterioration of the coupling efficiency due to the coupling misalignment is not a problem limited to the SHG device, but this is serious also for the case where a semiconductor laser and an optical waveguide device, e.g., an optical fiber, are secured on a sub-mount for optical coupling.